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Biology for Secondary Schools

metabolism can result into excess water nitrogenous wastes are removed from the
and salt in the body. The excess water bodies of living organisms in a form which
and salts in the body have to be excreted. is determined by the availability of water
Through excretion, organisms remove in the organism’s environment. In this case
waste products which are harmful to the animals living in terrestrial environment
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cells and organs to maintain homeostasis. which is characteristically dry usually
In animals, water is removed through three excrete urea or uric acid while organisms
physiological processes namely: urinary which live in water often excrete ammonia.
excretion, evaporation from the lungs Excretion regulates the movement of water
through breathing and sweating through and mineral salts inside and outside the cell
skin. Urinary excretion removes the largest membranes, therefore, maintaining osmotic
amount of water. In mammals, nitrogenous balance. The pH of the body fluid is also
excretory products are eliminated as urine regulated by excretion process by removing
which consists mainly of water, urea and excess hydrogen ions and acidic metabolic
salts. wastes. Therefore, without excretion, the
Excretion facilitates the removal of toxic equilibrium of these materials shifts and
wastes from the body which when allowed disrupts homeostasis and osmotic balance
to accumulate may damage cells, tissues, which can lead to serious health problems
organs and can also inhibit actions of many and ultimately death. Table 4.1 presents
enzymes involved in various metabolic the summary of various excretory products
pathways. It also helps in the regulation of produced by different organisms.
water content of the body fluids. Excretory

Table 4.1: Summary of excretory products produced by organisms
S/n Excretory product Source Excretory organ Type of organism
1 Carbon dioxide Cellular respiration Lungs, plant leaves and Animals and plants
stem bark via lenticels
2 Oxygen Photosynthesis Plant leaves and green Plants
stems (when light is
available)
3 Excess water Metabolic activities Kidney, skin and lungs, Animals and plants
plant leaves and stems
4 Bile pigments Breakdown of Liver Vertebrate animals
cholesterol and older
haemoglobin
5 Ammonia Deamination of Liver Most aquatic
amino acids animals
6 Crystals, gums, Metabolic activities Stem barks and leaves Plants
resins, tannins and
latex
7 Urea Metabolic activities Liver and kidney Animals
8 Uric acid Metabolic activities Kidney and Birds, reptiles
malphigian tubules and insects

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